Problem: In physics class, the girl to boy ratio is $8$ to $2$. If there are $16$ girls, how many boys are there?
A ratio of $8$ girls to $2$ boys means that there is a group of $8$ girls for every group of $2$ boys. If there are $16$ girls, then there are $2$ groups of $8$ girls each. Because we know the ratio is $8:2$ , there must be $2$ groups of $2$ boys each. There is a total of $4$ boys in physics class.